Guys.... For years, I was scarce-off by this talk of oscillation and instability associate with #26 linestage, without regulated filament supply, multiple chassis and so forth.

Kevin's talk of using a bench supply for the filaments jerk me up from my slumber to realise that I can use mine without resorting to invest further.
I've this Laboratory Power Supply ( 2 channels @ 30v@ 3A each) for charging lithium battery for my eBike.

So, I built one channel, linestage and B+ power supply all on a single chassis, just for a listen ..... expecting all hell to break loose with high pitch whistling, humming and heavy thumpping of my woofer!

Guess what, it was dead silent, sounds good and superb wholesome bass!

From the pics above, it was lest than 8 ins. away from my 80 watts monobloc power amp. Equally close is my CD player and the bench supply.

I have drafted the schematic as built . PLEASE advice what I should do and undo for it to sound better. THANKS!

Perhaps you just did a good job putting it together. The Lundahl plate load chokes were a great choice as they are less sensitive to external fields than some others I have tried, and you have also oriented them in a different plane from the power transformer which is a further help. ST glass 26 are significantly less microphonic in general than globe types. What sort of sockets did you use? (I noticed that teflon sockets oddly enough help with microphony transmitted through the chassis.)

The power transformer seems reasonably well removed physically from your chokes, and no question the supply design provides good ripple isolation. I use Rod Coleman CCS in some of my projects and they work very well.

How sensitive are your speakers and what is their LF cutoff? I use shield cans and need to with speakers with an LF cut off at 35Hz and 100dB efficiency.

__________________"To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine

I have played with a lot of DHT and the issues reported regarding hum etc can mostly be eliminated by implementing the filament supply correctly as well as placement of the power supply iron . Too high a filament voltage is usually the cause of microphony which does not seem to be an issue with the ST type 26 , although I have had no luck with globes . I doubt oscillation will manifest itself with a valve with such low gm as the 26 no matter how bad the layout is

Hello Kevin! You're my inspiration and it was your ground work that set my #26 PreAmp aloft.

As mentioned, my speakers are Altec 604-E Duplex of 101 dB sensivity.

I love those balloon valves and that's is exactly what I'm using.
Believes sockets are not Teflon but modern hard plastic.

Best part is, I don't even bother to isolate, cushion nor suspend it above /below the top plate.

But to be fair, if you tap the glass with a test pen, there's little microphonic but not on the top plate.

My power amp's 7F7 and 7N7 were much worse!

Ya... I've contact Rod to purchasing his regulators.

Happy Holiday to All.

Zekk

Zekk, Your layout work looks to be carefully considered, and neatly implemented - this will make a big difference to noise pick-up problems.

The lab supply must be pretty good - often they are too noisy for something like filament bias. But the lab supply will still try to null-out the music signal across the filament, because the filament is now in its voltage-feedback loop. You will hear the improvement in sound when you try purpose-designed current-driven heating, which addresses the problem. I responded to your message, but the email was rejected - please send your new email address.

Try both, I'd expect constant current mode to be better somewhat better. Measure the voltage across the filament to make sure you are in the ball park of 1.5V across the filament - some variation in voltage measured due to contact resistance is expected.

I have to my chagrin found dht filament heating to be quite sensitive to the quality of the filament supply and not in just obvious areas like ripple, voltage stability, and source impedance.. The wrong or badly designed filament supply can result in really bad sound.

__________________"To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine